Peel the potatoes and slice them as thinly as you can widthways, discard the ends.
Rinse the potatoes, before placing them in a large saucepan, covered in cold salted water.
Bring them to the boil. As soon as they start to gently bubble, set a timer for 5 minutes. Simmer for 5 minutes before draining and setting aside.
Melt the butter in a large pan. Add the cabbage and leek and fry for 3 minutes stirring regularly.
Stir in the garlic and fry for another minute.
Heat the oven to 200C fan
Make up the stock in a large measuring jug. Stir in the mustard, cream, Thyme and season with salt and pepper.
Tip half the potatoes into an oven proof dish. Spread half the cabbage and leek over the top. Pour half of the cream mixture over the potatoes.
Repeat with another layer of potatoes and cabbage and pour the last of the cream over the top so that the potatoes are just covered (You can add a splash of milk to bring the mixture up a little if needed)
Cover the top with grated cheese and bake in the oven for 30 minutes.
Remove from the oven, cover the dish with tin foil and return to the oven for another 15-20 or until the potatoes are tender.
Remove from the oven, take the foil off and leave to rest for 5-10 minutes
